This invention relates to controlling the rate of flow of polymer solutions. In one aspect the invention relates to equipment employed in connection with the injection of polymer solutions into wellheads. In another aspect this invention relates to a device which will allow one to control the rate at which a polymer solution is injected into a wellhead with a minimum adverse effect upon the polymer viscosity.
In the injection of polymer solutions employed in waterflood operations it is often necessary to exert some control over the rate of flow of the polymer solution into the wellhead. In certain situations it is also advantageous to be able to quickly change the rate at which the polymer solution is injected into the wellhead. A commonly used method of controlling the flow rate of fluids pumped into a wellhead involves controlling flow by use of a globe type valve. In the injection of certain polymer solutions, e.g., aqueous polyacrylamide solutions, the sudden pressure drop across the orifice provided by the setting on a globe valve causes the polymer to break up. When the polymer breaks up the solution becomes less viscous. H. L. Bildartz and G. S. Carlson in "Field Polymer Stability Studies", SPE Paper 5551 (1975) have shown that the shear degradation of polymer in conventional injection wellheads results in viscosity reductions of 26 to 41 percent and screen factor reduction of 13 to 54 percent. When using a globe valve to control the flow of such a polymer solution, more polymer must be employed to insure a specific solution viscosity. Employing additional polymer to compensate for polymer destruction is, of course, not economically desirable.
A conventional method for reducing the flow rate of a shearable polymer with a minimum of polymer deterioration involves the employment of a device known as a "choke coil". A typical example of "choke coil" involves three parallel 50-foot lengths of 5/16" copper tubing wound around a mandrel so that the complete assembly is only about 6".times.18" overall. Such a "choke coil" is capable of passing 880 bbl. of water per day with an input pressure of 1350 psi and an output pressure of 950 psi. If such a 400 psi pressure drop were provided by employing a globe valve or other variable orifice the polymer would be subjected to undesirable shearing. Generally when employing shearable polyacrylamide solutions abrupt pressure drops greater than 25 psi produces an undesirable amount of polymer shear. When a pressure drop of 400 psi is spread over the 50-foot length of the tubes in the "choke coil" the damage to the polymer molecules is minimal.
A shortcoming of such "choke coil" devices is the absence of a means of adjustment which would allow an operator to quickly and simply alter the amount of pressure drop provided by the device. When for some reason it has been necessary to change the amount of restriction of flow, operators have had to disconnect the "choke coil" from the injection line and either shorten or lengthen the coils in the device since each "choke coil" is designed to provide a particular set of flow conditions.
There are various situations where it is desirable to be able to vary the flow rate of the polymer solution more quickly and conveniently than can be done with a conventional "choke coil". For example, as polymer injection progresses the wellhead pressure can approach the formation fracture point. In order to prevent undesired fracturing it becomes necessary to reduce the flow rate of the polymer solution. Also, in waterflooding, it is recognized to be economically advantageous to inject polymer solution in the form of "slugs" of decreasing concentration. In order to insure certain flow rates the restriction of polymer flow must be varied as the polymer concentration is changed.
In response to the lack of adjustability of "choke coils", a flow rate controller for degradable polymer solutions, called a "packed column regulator", has been developed. The "packed column regulator" consists essentially of a vertical section of pipe packed with sand or glass beads. The flow through the device is regulated by changing the amount of sand or beads in the column. It should be evident that even with the "packed column regulator" a significant amount of alteration of the device and experimentation is required to obtain the desired flow rate of polymer solution. Thus there has been a need in this field for a device which can be quickly and easily adjusted to modify the flow rate of polymer injection solutions while producing a minimum of polymer degradation.